With a nearly 50-percent mortality rate, Vibrio vulnificus is the most deadly foodborne pathogen in the world, according to University of North Carolina at Charlotte Biology Professor Jim Oliver. And instances of infection in the U.S., however rare, are rapidly rising.

Fifteen years ago, there were 21 confirmed cases of Vibrio vulnificus and parahaemolyticus infections in the U.S. Last year, there were 193.

While infections from either of the pathogens are still rare compared with, say, Salmonella and Campylobacter, the incidence rate grew faster than any of the other five microbes tracked in the Centers for Disease Control’s 2012 Food Safety Progress Report. The vulnificus strain is responsible for 95 percent of seafood-related illness fatalities in the U.S., according to a 2013 study by Oliver and Joanna Nowakowska. Another Vibrio strain, parahaemolyticus, is milder, causing diarrhea, nausea, fever and chills, according to CDC.

Several studies have linked Vibrio’s quick growth rate with rising ocean temperatures, a critical condition favorable to the saltwater-based bacterium. Instances of Vibrio have started showing up in colder places where they were largely unheard-of before.

“Most notably, they’ve been [seeing cases] in places like the Baltic and Germany,” Oliver said.

While those cases usually involved Vibrio entering humans through wounds while they were swimming, a 2009 article by Oliver and Melissa Jones shows that about 93 percent of Vibrio cases in the U.S. manifest themselves in people who have consumed raw or undercooked oysters. Vibrio can also come from other undercooked seafood.

Vibrio doesn’t harm the oysters in any way, according to Rohinee Paranjpye, a research microbiologist with the National Oceanic and Atmospheric Administration.

“It appears to be a symbiotic relationship,” Paranjpye said.

Several post-harvest processing methods exist, which have varying degrees of success at killing Vibrio, but they have several drawbacks, said Chris Nelson, a trustee of the Gulf & South Atlantic Fisheries Foundation. One of the largest barriers is cost.

“There’s a huge barrier in terms of capitalization,” Nelson said. “Let’s say you needed a million dollars — and some of the post-harvest processing pieces of equipment are upwards of a million dollars — you have to be a certain size operation.”

“Probably for every dollar spent processing, it’s going to result in at least three more dollars on the dinner plate,” he said. “And probably even more than that because, if you make the additional investment, you’re going to want an additional profit.”

The least-expensive method, which involves freezing the oysters, is typically not used during the summer months when the water is warmer and Vibrio cases are more likely to rise, Nelson said. During the winter, when oysters are plumper, they can take the beating of post-harvest processing. But, when it gets warmer, the reproduction cycle takes much of the meat out of oysters, and processing can reduce the product’s quality, he said.

However, the costs of processing vary depending on several factors, including the method used, the market intended for the oysters, and whether the company in question is using its own material or paying somebody else to process their catch.

For instance, processing can actually reduce the price of shucked oysters, according to the ISSC’s report. Certain methods will help open oysters up, which almost cuts in half the amount of time it takes to shuck them. However, the report also showed that only 40 percent of oysters are sold to the shucked market, while 60 percent are sold to the half-shell market.

Nelson said some regulations exist for post-harvest processing, but they vary by location and time of year.

Cooking oysters can kill Vibrio as well. But, as long as ocean temperatures continue to rise, Oliver said Vibrio will continue to be a problem.